The present invention relates generally to acoustic transducers and, more specifically, to acoustic transducers for lesion feedback, catheter tip contact-monitoring, tissue thickness measurement, and pre-pop warning.
Cardiac ablation practitioners would like lesion feedback and contact monitoring from their thermal (e.g., radiofrequency or RF) ablation catheters. They would preferably also like to further know the thickness of target tissues and the proximity of organs to be avoided such as the esophagus, aorta, and lungs. Finally, some warning of pre-pop conditions would also be quite valuable. By “contact” we mean at least verification of intimate tissue contact of the ablating catheter tip, and even more preferably, also the measurement of the actual contact force involved. The contact force is of interest in order to avoid unintended tissue puncture and to guarantee good ablative results.
Electrically based RF ablation catheter lesion-feedback products now in development use indirect approaches comprising monitoring of electrical tip-coupling RF-impedances or parameters based on such tissue-coupling impedances measured at various RF frequencies including frequencies different than the RF ablation frequency. They essentially take advantage of the already existing electrical coupling of the tip to tissue to electrically deduce information about the lesion size. These are indirect approaches offering some additional value over simply monitoring the impedance only at the RF ablation frequency which as has long been the practice.
Proposed optical methods for lesion feedback include those disclosed by Biosense-Webster wherein the total integrated optical back-scattering of ablating tissue is monitored. In this approach, illumination light is directed into target tissues from a juxtaposed RF ablating catheter tip and, as lesioning proceeds, light is increasingly back-scattered from various depths of the forming lesion volume giving an indirect but still useful indication of total lesion volume based on total integrated back-scatter. One potential disadvantage of optical techniques, other than the cost for a multi-fiber fiber-optic solution, is that tissue surface charring can partially blind the probe by blocking all light penetration at the tissue surface. As such, this must be avoided and/or accounted for.
Determination of catheter ablator tip tissue-contact has long been done by (a) monitoring the electrical contact impedance at the RF ablation frequency, and possibly also (b) monitoring the apparent deformed shape of the catheter in an X-ray fluoroscopy image in addition to (a). More recently, a number of optical methods utilizing optical fibers have been suggested and are being developed such as that of Enclosense Inc. wherein optical fibers are used to monitor tip displacements and therefore tip forces. Approaches which utilize 3 or more such optical fibers plus dedicated LEDS and photodiodes can become expensive to manufacture and do not leave much room for other important catheter components such as catheter steering wires and fluid lumens. They also will have a somewhat higher failure rate, higher manufacturing cost, and lower manufacturing yield given the large number of added components. However they can work.
We utilize one or more pinging acoustic transducers mounted in or adjacent the catheter tip to acoustically detect lesion volume and tissue-contact if not also tissue contact force. Unlike the above optical backscatter approach, the acoustic pulse-echo approach also allows a user to discern the lesion state at specific depths because time-delay range data is available. This also allows for direct measurement of tissue thickness or organ proximity.
One might ask why not instead simply utilize ICE probes (intracardiac echo ultrasonic phased-array imaging probes) to image all of the ablation catheter, the lesion(s), and the heart chamber(s). There are several reasons for this including the following. (a) It is a separate additional fairly expensive device. (b) Currently available ICE imaging catheters image 2D slices. It is not easy in a beating heart to find and remain aimed at the ablating catheter tip given that the 2D ICE image plane must be aligned perfectly with the ablating tip. (c) During ablation the lesion is under the ablating catheter tip and you cannot see through the tip from the blood pool. ICE imaging catheters are still eagerly employed by many practitioners today for general visualization of target anatomy but they do not yet provide useful lesion information for the above and additional reasons.